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How to calculate earth fault loop impedance value (Zs)

the Breaker Earth Loop Impedance (Zs) is calculated as follows; Zs = Uo / Ia

Where Uo is the voltage to earth and Ia is the current to trip the breaker in a given breaker disconnection time.

Usually we consider two breaker disconnection times: 5 seconds and 0,4 seconds, depending of the application.

Disconnection time of five seconds is used for fixed equipment. i.e. a machine in a factory or a fixed appliance (electric oven) in a house.

Disconnection time of 0.4 seconds for portable equipment. i.e. power drill or electric lawn mower. This disconnection time is usually used for final circuits (MCB's

A - For a 5 seconds disconnection time
1 - Get to know the voltage to earth (Uo)

2- Determine Ia, see the breaker tripping curve, to find the cuurent needed to trip in 5 seconds (see example on the attached file “Earth Fault Loop Impedance Summary.ppt”, slide 2 and 3)

3- Calculate Zs = Uo / Ia


B – For a 0,4 seconds disconnection time

1- Breakers with electronic trip units – Micrologic (Compact NSb, Compact NSX)

Zs = (Voltage Phase to earth) / (In x Io x Ir x Isd x 1.1)

On some trip units “Io” doesn’t exist, in this case consider Io = 1.

2- Breakers with thermal-magnetic trip units (Compact NS, Compact NB, EasyPact EZC, Compact NSX)

Zs = (Voltage Phase to earth) / (In x Io x Ir x Isd x 1.2)

On some trip units “Io” doesn’t exist, in this case consider Io = 1.
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